1. Technical Field of the Invention
The present invention relates to a method and apparatus for optically reading out information from an object, and in particular, to the method and apparatus that use an image-forming lens for imaging a reflected light from an information code and an optical sensor for receiving an image from the image-forming lens.
2. Description of the Related Art
A code reader such as a bar-code reader has been known as a device to read out pieces of information from a bar code attached to objects. To read the information from the barcode, a device called bar-code reader is required.
A handy type optical information reading apparatus is categorized as one of such devices and the apparatus is configured to read information such as a bar code or a two-dimensional code. Such an apparatus usually has a reading unit comprising an optical sensor, an optical image-forming means having an image-forming lens, and lighting means within a portable main unit. An object from which information is optically read (for example, a bar code, hereinafter referred to as “object”) is illuminated by the lighting means through a reading window located at the end of the main unit. Thus, an image is formed by the optical sensor via the optical image-forming means by incorporating a reflection light of the object through the reading window.
A reading distance (i.e., distance from the optical information reading apparatus to the object) for reading an optical information is determined by optical characteristics (mainly by the depth of focus) of the optical image-forming means in the optical information reading apparatus with a certain range of distance. Therefore, the user of the optical information reading apparatus conducts the read operation by moving the reading apparatus (i.e., reading window) to an appropriate read position (i.e., best focus position) of the object. Recently, on the other hand, to widen the range of the readable distance, an optical information reading apparatus having an auto-focus unit has been developed, which can change the depth of focus by shifting a lens system. For example, the auto-focus unit which changes the depth of focus by shifting the lens system along its optical axis has been developed by the combination of a ball screw and a stepping motor (for example, Japanese Patent First Publication No. 7-319990).
However, the above auto-focus unit requires a fairly complex mechanism and control for the unit in spite of the advantage of stepless change of the depth of focus. Especially, the driving unit of the lens system shifting along a straight line becomes complex, thereby resulting in a large unit. Furthermore, since a moving part of the driving unit is heavy, the response speed becomes slow, resulting in disadvantage of the unit.